The OFDM technique has been successfully deployed in indoor wireless Local Area Network (LAN) and outdoor broadcasting applications. OFDM reduces the influence of inter-symbol-interference with a complexity less than that of typical single carrier adaptive equalizers, and has been found to work well in multipath fading channels. These and other advantages render OFDM as a strong candidate for use in proposed 4 G mobile communication systems.
Under a frequency selective fading channel each sub-carrier is attenuated individually. The resultant sub-channel frequency functions are frequency-variant and may also be time-variant, hence adaptive modulation may be appropriately applied to improve the error performance and data throughput in an OFDM modem by assigning different transmission power and/or modulation and coding schemes to different sub-carriers. See, for example, T. Keller and L. Hanzo, “Adaptive Modulation Techniques for Duplex OFDM Transmission”, IEEE Trans. on Vehicular Technology, Vol. 49, No. 5, September 2000, pp. 1893-1906, and B. S. Krongold, K. Ramchandran and D. L. Jones, “Computationally Efficient Optimal Power Allocation Algorithms for Multicarrier Communication Systems”, IEEE Trans. on Communications, Vol. 48, No. 1, 2000, pp. 23-27.
One fundamental issue that arises when attempting to deploy adaptive modulation is the determination of the number of bits and/or power to be loaded into each of the sub-carriers. One known solution is referred to as a “parameter optimization approach” that formulates the bit/power loading issue as an analytical parameter optimization problem. Families of analytically-derived bit/power loading algorithms to maximize a performance criterion subject to one or more constraints for an un-coded OFDM system are readily available. Unfortunately, channel coding, which is frequently employed to combat fading, may be difficult to incorporate in such an analytical approach. In fact, there is little literature available that concerns the optimization of data throughput in a coded OFDM system. As a result, the issue of sub-carrier power loading in a coded OFDM packet-based modem, to improve or maximize data throughput under a fading channel, is an unresolved problem in those OFDM systems that employ channel coding.
It is known to employ heuristic methods, or to employ analytical means under un-coded conditions. One known approach is to treat the issue as a parameter optimization problem and to then employ analytical optimization techniques (see, again, B. S. Krongold, K. Ramchandran and D. L. Jones, “Computationally Efficient Optimal Power Allocation Algorithms for Multicarrier Communication Systems”, IEEE Trans. on Communications, Vol. 48, No. 1, 2000, pp. 23-27). Typically such approaches seek to maximize the rate (bits/OFDM symbol) subject to a Bit Error Rate/Symbol Error Rate (BER/SER) bound and other constraints (e.g., power). However, it can be shown that this approach does not necessarily optimize the net throughput, especially in a packet-based system. Further, channel coding would be very difficult to incorporate in such an approach.
For instance, the above-referenced Krongold et al. proposed a Lagrange bisection solution that maximizes the rate (bits/symbol) subject to a total power constraint and a fixed error probability bound. An additional practical constraint is that the rate should be an integer number of bits/symbol. As was noted, however, channel coding, which is frequently employed to combat channel-induced errors, may be difficult to incorporate in such an analytical approach. Meanwhile in a packet-data based system with channel coding, it may be more desirable to maximize the net data throughput (also known as “goodput” in some literature) defined as (1-PER)*data_rate, where data_rate is the actual data rate in packets/symbols per time unit (or other normalized values), rather than the raw data rate. However this is difficult to perform analytically.